1. Often the way companies are structured requires specific teams and processes to exist and unwinding that is not easy.
2. It can be very hard to identify which people are actually critical. If you get it wrong it’s a big problem.
3. Additionally, firing a large percentage of employees will create a very negative feeling in the remainder so they might leave as well or their work quality will suffer.
4. Customers may lose faith in the business as a result and switch. Something like Atlassian has many competitors.
Are there any specific examples of this being done successfully?
The trick for this being successful is most of the time by challenging the processes themselves, rather than the employees. Employees aren't useless, but process are. The reason you need 500 people rather than 250 or 100 or 50 to do something might be simply because you said so somewhere or sometime.
So, companies should review and change processes BEFORE reducing headcount. But alas, companies don't ever do it, because it requires introspection, egolessness and actual work, the sorta stuff that is rare in upper management.
It also may reduce the need for middle and upper management.
In fact, if you give people some confidence that they won't be fire and instead you will use their work on some more valuable action, ICs will line up on your door with ideas on how to cut their processes. But management won't.
Amusingly, the best "firing" in one of those was the Dean of the University that had a seven-figure salary. The board replaced him with an administrator and with someone else from the faculty, and both were more qualified, but costed 1/10 of the price (still a huge raise for them).